The present invention relates to a thin film transistor liquid crystal display (TFT LCD) device, and more particularly to a reflective or transflective TFT LCD device having multi-layered pixel electrodes connected to drain electrodes with interposing an insulating layer therebetween.
TFT LCD devices are generally classified into a reflective TFT LCD device using a reflective layer as pixel electrodes, a transmissive TFT LCD device using transparent pixel electrodes, and a transflective TFT LCD device using a reflective layer having a transmissive region in a portion of a reflective region as pixel electrodes, according to reflectance or permeability of pixel electrodes. In the TFT LCD devices, to supply voltage for controlling arrangement of liquid crystal to the pixel electrodes, drain electrodes of thin film transistors formed in each pixel are connected to the corresponding pixel electrodes. The pixel electrodes are generally connected to the drain electrodes through via holes formed in an interlayer insulating layer.
In a transmissive TFT LCD device, pixel electrodes use indium oxides to form transparent electrodes. However, this material may cause a problem that oxidizes wires of aluminum (Al) to form insulating oxides and thereby hinders in supplying voltage to the pixel electrodes. Therefore, in the transmissive TFT LCD device, drain electrodes are formed of a single layer of metal such as chromium (Cr), or a two-layered conductive layer having an Al-contained metal layer and a molybdenum tungsten (MoW) or Cr layer formed on the Al-contained metal layer.
In a reflective TFT LCD device, pixel electrodes usually use aluminum neodymium (AlNd). In this case, materials forming drain electrodes are also limited. Referring to FIG. 1, a pixel portion of the reflective TFT LCD device using an easily oxidized metal such as Al as source and drain electrodes 21, 21xe2x80x2 is illustrated. On the source and drain electrodes 21, 21xe2x80x2, a protecting layer 23, for example a photo-sensitive organic insulating layer is disposed. The protecting layer 23 has via holes for connecting the drain electrodes 21xe2x80x2 to the pixel electrodes 27. Therefore, in an exposure, development and cleaning process of photolithography for forming the via holes, if developer or detergent of strong oxidant contacts the drain electrodes 21xe2x80x2 through the via holes, the upper layer of the drain electrodes 21xe2x80x2 may form insulating oxides 25. The insulating oxides 25 increase contact resistance between the drain electrodes 21xe2x80x2 and pixel electrodes 27.
To solve the problem, an upper layer 212xe2x80x2 of the drain electrode 21xe2x80x2 can be formed of metal such as MoW that is resistant against oxidation, as shown in FIG. 2. However, in this case, battery effect, like inside a chemical battery, can be occurred due to difference of electro-negativity between the upper layer 212xe2x80x2 of the drain electrode 21xe2x80x2 and an Al-containing reflective layer forming the pixel electrodes 27. For example, due to corroding by the battery effect, gaps 29 similar to spike phenomenon generating at the interface between a silicon layer and an Al layer can be formed at the interface between the upper layer 212xe2x80x2 and the Al-containing reflective layer. Also, as a portion of the Al-containing reflective layer around the gaps 29 falls, the Al-containing reflective layer can generate cracks 31 around the via holes. These gaps 29 or cracks 31 cause a problem increasing contact resistance between the pixel electrodes 27 and the drain electrodes 21xe2x80x2.
Generally, the battery effect increases in proportion to the difference of surface area and electronegativity between two metal layers. Accordingly, the drain electrodes 21xe2x80x2 that usually has relatively very small surface area compared to the pixel electrodes 27 enforces the battery effect more, thereby increasing the contact resistance between the pixel electrodes 27 and the drain electrodes 21xe2x80x2 more.
To solve the battery effect, it can be considered to use a MoW or Cr layer as reflective plates or pixel electrodes 27. However, such a choice deteriorates reflectance and conductivity of the pixel electrodes.
Accordingly, a new TFT LCD device, which can prevent increase of contact resistance due to the battery effect or surface oxidation at the interface between the pixel electrodes and the drain electrodes with maintaining highly reflectance and conductivity, is required.
It is an object of the present invention to provide an improved TFT LCD device that can prevent battery effect at the interface between pixel electrodes and drain electrodes, while maintaining high reflectance and conductivity.
It is another object of the present invention to provide an improved TFT LCD device that can prevent insulating oxides at the interface between pixel electrodes and drain electrodes, while maintaining high reflectance and conductivity.
It is other object of the present invention to provide an improved TFT LCD device that can prevent contact resistance increase at the interface between pixel electrodes and drain electrodes, while maintaining high reflectance and conductivity.
These and other objects are provided, according to the present invention, by a TFT LCD device having pixel electrodes formed of a multi-layered conductive layer. Preferably, drain electrodes are composed of multiple layers, and the most upper layer of the multiple layers is composed of a metal layer that is strongly resistant against oxidation. Also, the multi-layered conductive layer is composed of two-layered conductive layer having a lower layer of metal that has small electro-negativity difference between itself and the most upper layer of the drain electrodes and an upper layer of Al-containing metal.
In the present invention, the lower layer of the two-layered conductive layer is preferably composed of the same material as that of the most upper layer of the drain electrodes, for example one selected from a Cr layer and a MoW layer. The Al-containing metal usually uses pure Al or AlNd. Accordingly, the two-layered conductive layer is formed by depositing the lower layer of one selected from a Cr layer and a MoW layer and the upper layer of Al-containing metal and then patterning them.
It is noted that the multi-layered conductive layer is not limited to the two-layered conductive layer. To reduce the battery effect efficiently, if necessary, an intermediate metal layer can be interposed between the lower and upper layers of the two-layered conductive layer.
The multiple layers of the drain electrodes usually use metal having a high conductivity to prevent a signal voltage drop due to the data line resistance. Also, the drain electrodes are formed of the same conductive material as that of the data lines connected to source electrodes. Accordingly, the multiple layers forming the drain electrodes are preferably composed of a two-layered layer having an Al layer for increasing conductivity and a Cr or MoW layer strongly resistant to oxidation formed on the Al layer, or a three-layered layer having an intermediate Al layer and an upper and a lower Cr or MoW layers formed on and under the intermediate Al-contained layer to prevent spike phenomenon due to silicon layers over an active area. When the drain electrodes formed along with the data lines are of the three-layered layer, the MoW layer as the lower and upper layer preferably is better than the Cr layer since it is easy to be patterned along with the intermediate Al-containing layer.
According to the present invention, since the upper layer of the drain electrodes and the lower layer of the pixel electrodes are formed of same material or metals having small differences in electro-negativity, the battery effect therebetween can be ignored. Also, in the two-layered conductive layer of the pixel electrodes, the upper layer and the lower layer are concurrently formed to have the same surface area by means of same patterning process. This may result in difference in electro-negativity, but battery effect can be prevented. Thus, at the interface between the pixel electrodes and the drain electrodes, the battery effect is considerably reduced and the spike phenomenon or cracks is prevented.
Also, since the upper layer of the drain electrodes is composed of a metal layer strongly resistant to oxidation, insulating oxides are not formed on the upper surface thereof even though it is exposed to oxidizing material such as developer or detergent, and thereby preventing contact resistance increase.